1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) radio communication system, such as wireless private branch exchange (PBX) and microcellular mobile communications, particularly to a CDMA radio communication system in which constitution of an antenna in a base station is improved.
2. Description of the Related Art
In digital communication systems, there are three basic multiple access schemes, CDMA, frequency division multiple access (FDMA), and time division multiple access (TDMA). Since CDMA exhibits great advantages about privacy communication function and an extensibility of system construction over FDMA and TDMA, it will come into use widely in portable radio telephone systems and in microcellular mobile communication systems.
As is well known, in the CDMA systems, the spectrum of signals to be transmitted is spread by pseudo-random noise (PN) codes. Under a condition that the spread bandwidth of the transmitted signal in the spectrum spread system is sufficiently wider than a correlation bandwidth in the transmission path which will be accompanied with multipath fading, the fading can be reduced by using a path diversity system such as a RAKE system or a post detection integrator (PDI) system. Such a RAKE system is described in "Introduction to Spread-Spectrum Antimultipath Technics and Their Application to Urban Digital Radio", by George L. Turin, Proceedings of the IEEE, Vol.68, No.3, Mar. 1980 in detail.
At the receiving end of the CDMA system, a correlation between the received signal and a PN code used for spreading the signal to be transmitted in the particular transmitting end will be calculated by a correlator, and thus output of the correlator will have a resolution of time which corresponds to a symbol period of the spreading code. The output of the correlator is applied to a delay line having a plurality of (TDL; tapped delay line). A plurality of outputs appear at certain taps of tile delay line depending upon the different delay amounts caused by multipath are, according to the RAKE system, weighted by values which are proportional to strength of the respective paths and combined together (maximal-ratio combining). Since there is no correlation between the variations of the scattered waves with respect to time, although each of which will have large fading, this combining results in variation of width of the combined signal to decrease. Thus, reduction of the signal level due to the fading can be reduced.
In general, a correlation bandwidth of the fading differs depending upon multipath environment, about several hundreds KHz to one MHz in the outdoors and about 10 MHz or more in the indoors. On the other hand, the spread bandwidth of CDMA is restricted to several MHz, e.g. 2.0 MHz, or less due to technical problems such as tile limitation of high speed digital signal processing or to the limitation of frequency bandwidth allocated for the system. Thus, in a particular environment such as indoor communication or microcellular radio system in which the correlation bandwidth is wider than the spread bandwidth, the output from the correlator will appear only at one or two taps of the delay line resulting in very small fading reduction by the path diversity function. Namely, in such an environment, the output signal from the receiver will be deteriorated by the short period fading such as Rayleigh fading.
In case that CDMA, especially direct sequence spread spectrum CDMA which will be appropriate to a digital system, is adopted to the mobile radio communication, a near-end to far-end interference, so-called near-far problem, will occur. When a mobile station close to a base station can mask the received signal at the base station, this interference occurs so that the signal from the far-end mobile station is unable to be received by the base station at the same time. This is caused because, in the CDMA cellular mobile communication system, all the stations in the same cell area transmit on the same frequency band. In order to reduce such the near-end to far-end interference, it is necessary to use in the mobile stations a reverse-link power control for controlling transmitting power at the mobile stations so that the power strength of the signals, from all tile mobile stations in the same cell area, received at the base station are the same. Such power control is known by for example "Overview of Cellular CDMA", by William C. Y. Lee, IEEE Transactions on Vehicular Technology, Vol.40, No.2, May 1991.
If the variation of the signal strength is due to only long period variation which will vary depending upon path loss of the received signal given as a function of the path length or upon the degree of shadowing of structures surrounding the stations, the reverse-link power control in the mobile stations is not necessary to be performed in high speed and thus this power control can be easily performed by an open-loop power control system. However, if the aforementioned short period fading is strong, the reverse-link power control should be performed so as to reduce not only the long period variation but also the short period fading.
As is described in detail, although the CDMA systems would in general have the path diversity function at the receiving end to reduce multipath fading, in case of short-distance communication systems such as wireless PBX communication systems or microcellular mobile communication systems, this path diversity due to the spectrum spreading may be ineffective causing the output signal from the receiver to be deteriorated by the short period fading. In order to reduce such short period variation, the reverse-link power control at the mobile units is required to operate rapidly. However, high speed compensation by means of such reverse-link power control will be very difficult and thus, by the power control at the mobile units, sufficient reduction of the short period fading will be unexpected.